Japanese Patent Publication No. 59-37196 has disclosed an automatic screw driver with a screw supplying mechanism comprising a motor-driven screw driver body, a rotating bit an end portion of which is magnetized, a suction guide which surrounds the rotating bit and extends in the axial direction of the bit, a catcher body surrounding an end portion of the suction guide, an outer shell fixed on the upper portion of the catcher body, an inner shell which is fixed on the driver body and engageable with the outer shell, a first chamber and a second chamber formed inside the outer shell in response to a sliding movement of a piston portion which is slidably fitted in the inner shell so as to contact the inner surface of the outer shell, a mechanism for supplying alternatively fluid to the first and second chambers, a screw supplying mechanism communicated with an aperture which is formed at a distal end of the catcher body. The suction guide is slidably mounted in the inner shell in the axial direction of the bit. Further, when a pressure is applied in either the first chamber or the second chamber, the outer shell is moved toward the driver body relative to the inner shell and the suction guide.
The conventional driver with the screw supplying mechanism as disclosed in the publication is constituted such that a magnetic screw to be supplied to the tip end of the rotating bit is sucked by cooperation of air suction force and magnetic force of the screw and screwed into a tapped hole. In such a case, the screws to be supplied to the catcher body which surrounds the tip end of the rotating bit, namely the distal end of the suction guide, are automatically delivered one by one by air pressure supplied through an air pipe. When the screw is delivered to the catcher body, a switch of the motor-driven screw driver is turned on and the rotating bit starts to rotate. At the same time, the outer shell is moved toward the driver body by air pressure relative to the inner shell and the suction guide which are secured on the driver body. To this end, the screw is sucked and retained at the tip end of the rotating bit by cooperation of the air suction force and the magnetic force. The outer shell is further moved toward the driver body relative to the inner shell and the suction guide so that the tip end portion of the rotating bit projects by a sufficient length from a lower-cylindrical portion of the suction guide while the screw retained at the tip end of the rotating bit is allowed to rotate. Then, the screw is retained at the tip end of the rotating bit only by the magnetic force. Therefore, the screwing operation of the driver is performed in such a state.
As understood from the above description, the conventional driver with the screw supplying mechanism is regarded as advantageous from a point of view that the mechanism having a relatively simple structure serves for supplying the screws one by one preparatory to an automatic screwing operation.
However, if a screw made of non-magnetic material such as aluminum, brass, plastics, ceramics and the like is supplied, the screw is not sucked or retained at the tip end portion of the rotating bit in such a state that the tip end portion thereof projects by a sufficient length from the lower-cylindrical portion of the suction guide upon the screwing operation of the driver. To this end, the screwing operation is not performed. Thus, there has been a disadvantage that the screw made of the non-magnetic material is never usable for the conventional driver with the screw supplying mechanism. Further, even though the screw made of magnetic material is supplied, the screw is unstably retained at the tip end of the continuously rotating bit only by the magnetic force, resulting in causing looseness and falling-off thereof from the bit end.